Since heat caused the plastic to melt and stick to the shells in the
first place, have you considered just boiling the shells?  The plastic
bag pieces should float to the top.
 
I have faced the problem many times by getting a plastic wrapper too
near the toaster, electric frypan, or deep fryer.  However, since I
couldn't boil these items, I never successfully removed any.  It just
wore off over a period of time.
 
Sylvia Edwards
Huntsville, AL