Smoke from my iPhone

You were doing what? You have likely destroyed your phone. Apple does NOT replace the back glass on any iPhone as the glass is fused as you've figured out since you were using a heat gun on your phone.

You should immediately cease using the phone and recycle it so it can't blow up in your hands.

There's a good reason Apple does NOT replace the back glass on these phones. This was a most unwise thing to do on your part.

Congratulations. Your phone is now scrap. Toss it in the recycle bin,

Would have been less expensive to get the phone replaced by Apple...

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With all sorts of glues, adhesives, tape and other volatile materials inside any consumer electronics, applying heat directly is not only going to produce a bad smell, it can also generate toxic fumes. Not a good idea.