Let's assume a world where humanity managed to stabilize the climate again and maybe even reversed the effects of the current climate change. (So that we can ignore the current global warming in this question.)
In this world humanity grows further and builds more machines and electronic devices for an ever increasing standard of living. All of them are extremely optimized to have minimal energy consumption and such but no matter what we do each device (and each living being) produces some waste heat even if it is not much.
How realistic is it that in this world the waste heat will ever contribute significantly to the temperature of earths atmosphere?