In general Traveller and Old School Space Opera ships don't have them. It messes their lines up. It makes them look too ... primitive. If you ask me the lack of heat radiators makes a ship as unbelievable as a reactionless or FTL drive. I think we'd find away around Newton and Einstein before we find a way around Old Man Entropy. Just removing heat from a machine's operation changes way more than the aforementioned drives.
FTL and grav drives get you to the action. Then you can basically forget about them in a story. But heat is a necessary inevitable by product of any energy production or work. Take Star Trek's phaser, capable of annihilating a man sized quantity of nearly anything. A quick stop at the Atomic Rockets website's Boom Table indicates that it requires 3,000,000,000 joules of power to vaporize a human being (leaving behind not even a skeleton in a space suit).
(The Enigmatic Aliens got way less Earth ships nosing around when they started using these for markers instead of buoys that looked like multicolored cubes)
For the less science inclined (like me) that little bitty phaser put out the equivalent of 771 kg of TNT. Now assuming that ray gun is 99% efficient (which puts us firmly into the category of unobtainium, possibly with some handwaving involved as well) then you have waste heat equivalent to 8 kilograms of TNT coming off that phaser. Note that I'm leaving the question of how you get that much power in a phaser alone.
Nonetheless you can have all kinds of fusion powered dinguses running without baking people.
Back to the ships. In the old Digest Group Publications Starship Owner's Manual some nod was made to this. The Jump drive was described as a large fusion reactor optimized to produce a lot of power quickly. Most of the fuel for jump was used as coolant to keep the ship from melting. It made more sense than a Scout ship generating the equivalent of a 450 megaton nuke going off (again I checked the Boom Table, found the energy generated by fusing a kilo of deuterium, multiplied it by 20,000 and then looked up the final result o the Boom Table anyone wants to check my math feel free.)
This would require about ten thousand times as much hydrogen as the fuel required for a jump 2. So not a lot of that hydrogen is being fused or the Heat Radiator elves are awfully busy. We're looking at a Traveller ship actually fusing a few hundreds of a percent of their jump fuel.
(Fortunately Mongoose Traveller straightened us all out by deciding the hydrogen was used to maintain a bubble in jump space keeping the ship and crew intact. No more mention of nasty heat.)
But let's say that yes indeed Traveller ships vent hydrogen prior to jump to keep the heat down. Let's further say that heat radiators are an option for the enterprising merchant or Scout.
Heat radiators replace hydrogen fuel used for jump on a one for one basis. Up to 50% of a ship's jump fuel can be replaced by radiators. radiators cost 100K credits per ton. They take one turn to deploy or retract in combat and can't be operated at acceleration greater than 1g. If they are deployed on a ship accelerating greater than 1g roll a die each turn. If the die roll is less than the acceleration of the ship the radiators are destroyed.
In combat radiators are quite fragile. On any hull hit they are destroyed on a roll of 7+. In case of a misjump they are also destroyed on a 10+.
Obviously a ship can extend its range with a radiator. Also they can reduce costs when they have to buy fuel. You don't want them on a combat vessel though if you have enough fuel to make jump at the normal cost you can without deploying them.
It's harder to figure out heat radiators in a Space Opera game, especially if it includes defensive shields. Any forcefield that could deal with a nuke going off nearby or a gigawatt laser could probably also be tweaked to radiate a ship's waste heat which explains why the Star Warsy/Treksy ships don't resemble windmills. If a ship wants to double dip it could extend a heat radiator beyond a forcefield giving the ship a plus to damage or movement at a corresponding minus to armor class.