To be honest, using a heat gun is probably the safest way. While yes, it can give off fumes if you scorch it, you're going to notice the plastic overheating long before you get fumes in a high enough concentration to actually cause harm - as long as you're not working in something the size of a phone booth or a port-a-'fresher. Boiling water, on the other hand, you do have a significantly higher risk of burns as opposed to a heat gun. While these can be mitigated with proper safety gear - namely work gloves - a heat gun is more easily controlled.