How can I reduce the heat produced when running resource heavy applications for a long time?
Firstly, probably worth checking that kind of temperature isn't going to damage your machine. This might be helpful at working that out: http://www.buildcomputers.net/cpu-temperature.html

As you'd likely know, processors and GPUs have an optimal temperature they run at which is regularly around 50 degrees C. Yours is well over that.

It'd be easier to suggest things if we had an idea of the spec and model but these a few general things

Few things to try:


Ventilate - ensure that the air vents are clear - this includes dust and things on your desk - it goes without saying that while a laptop has the word lap in it, it doesn't mean use it on your lap. Lots of laptops I've seen with heat damage are from doing things in bed with the sheets blocking the vents.
Ensure your fans are operating currently - if they don't sound like they are running at full speed when the CPU is at 80, then your fans or sensors aren't working. To that end, if you are good with physical tools, take the machine apart and clean the fans out with compressed air 


If the issue persists take it to an authorised repairer. There could an issue with the thermal paste, and that kind of thing should be fixed by a professional. 

If this still doesn't resolve the issue it's likely that you have a machine that is under powered to your needs and you are working it too hard. In other words, it might be time to upgrade to an iMac Pro later this year