Filters and stripping columns. It’s not that hard to do and if you have a clean source of plastics (not mixed with garbage) then it’s basically all just hydrocarbons with a little bit of nitrogen. Carbon + oxygen + hydrogen = CO2 and H2O which while we don’t need more CO2 it’s significantly better then generating that same energy from oil as it is cleaning up our waste vs extracting more from the ground.

Its a lot harder when you do trash incineration as that has a lot more nasty stuff in it but plastics are easy