[danw]

I, too, caught an escaped plastic bag on my stainless steel pipes last year.

Tried cold water on hot pipes. Nothing!

Tried nail polish remover (acetone). Nothing!

Tried EZ Off oven cleaner. Nothing!

The problem is that bag material (polyethylene or polypropylene) is chemically inert. Nothing cuts it.

Finally...
I heated up the pipes (the old fashioned way, I drove the car around the block), then scraped off most of the heavy stuff using a wooden paint stick with a sharpened end. Repeated the process until I was mostly down to bare metal, then used Borla Exhaust Cleaner and fine bronze wool and worked my way in from the edges of the plastic that was left. Done! Clean! Let's go for a drive!