A plastic coated bib or zip top bag gone astray probably won't melt in a dryer unless you're using the high heat setting, but if it does, it won't cause serious damage. Today's dryers are painted with a synthetic enamel coating that holds up well to heat and wear. However, harsh abrasives or acidic cleaners may etch or scratch the surface, causing more damage than the melted plastic.
Run the dryer on a regular setting for 20 minutes to soften the plastic. Scrape the plastic off with a wooden or plastic spatula or paint scraper, taking care not to damage the dryer's interior.
Apply 1 to 2 tsp. nail polish remover to a clean cloth, and rub the plastic to dissolve it. Rub the area with a damp cloth to rinse the nail polish remover.
Mix 1 tsp. liquid laundry soap with 1 cup water. Dip a clean cloth in this solution, and rub the area again. Rinse well with a cloth dipped in clean water.