How do you get melted plastic off a fireplace screen?

put a piece of paper on the plastic and iron it very well and very hot.
If it's a metal screen, reheat the plastic until it melts again and wipe it off..quickly!
Assuming the screen is non-combustible (and it had better be!), you can use a propane torch to burn off the plastic. Try not to overheat the screen, as you will probably discolor it if you do.
place on top of your BBQ grill for seconds at the time til its all gone grill will burn it from itself if you let it burn out
If you're talking about a flat glass screen, use a single edge razor blade, carefully, to scrape it off the glass.
Use a hairdryer to melt it off.