The way to solve this problem is with heat exchangers positioned inside the exhaust pipes or smokestacks. As the hot exhaust gases drift upward, they brush past copperr fins with water flowing through them. The water carries the heat away, back into the plant. There, it might be recycled directly, maybe warming the cold gases that feed into the engine or furnace, saving the energy that would otherwise be needed to heat them up. Or it could be put to some other good use, for example, heating an office near the smokestack