Gas Dryer - burning plastic smell

Hi all - gas dryer putting out a burning smell - looked inside drum and it looks like burnt clear plastic next to the vent holes on the left of the drum. Is that where the heat comes into the dryer drum? Where did the plastic come from? I am wondering if it was simply a piece of plastic tape or something that adhered to the warm surface and melted - is this possible?

The vent looks a little brown itself - is that normal for a gas dryer that is 15 years old?

thx