Adjusting graphics cards settings in Mojoave to reduce overheating with external monitor (MacBookPro2018)
The 2018 15" MacBook Pros all have dual-GPUs. The AMD GPU is required to be active when an external screen is connected. There is no way to use the internal (Intel) GPU for external screens. 

Fan noise is one of the main nuisances of the recent MacBook Pros in my opinion. When I get too frustrated, I sometimes use Turbo Boost Switcher, which helps a bit. Volta is a similar App, but does not seem to work with 2018 MacBooks. Both Apps deactivate the CPU's Turbo Boost mode, which means less speed, but also less power consumption and less noise.

Update: I forgot to mention AppTamer; another tool which often saves me some noise.