We do not have food like that too often but when we do we simply follow the directions on the food packaging and microwave the food, I do use my microwave for a fair few things especially in the summer so as not to add more heat to the house.
Every now and then when we do buy them horrid scary frozen meals with plastic on top that you have to pierce with a knife or fork, it will be 3 or 4 of those meals that gets microwaved one after the other and we have never had an issue or smelled any fumes/melted plastic.
Any left overs that are wanted warmed in our house generally get put into plastic microwave containers (not the cheap ones as they do smell and I can't stand them) or on the plate covered in gladwrap and microwaved.
I do not believe the microwave heats the plastic- it is more the heat from the food that heats the plastic as far as I am aware.
I always have the exhaust fan on whenever anything is heated (including just water in the kettle) in the kitchen- it is just habit for me to hit the fan on then start making what I am making.