Depending on how big the blob of plastic is
I've used the following method
As I've had to remove the melted remains of a ladies boot heel from a silencer before
Heating up the plastic with a heat gun or hair drier & slicing off as much as possible, is a good start
For the residue, wait till it's hardened again, then adopt the suggestions above
A fine "suede" or brass wire brush (softer than the chrome) will get the last bits off
I find it's best to do this when the exhaust is hot
*
Final polish with Solvol or chrome cleaner of your choice & job done
* Chrome is a very hard but quite porous metal.
Minute particles of plastic will be embedded into the surface.
Getting the silencer hot will expand the surface and mke it easier to dislodge any remaining deposits