In my limited experience replacing tubes, I've used the heat & yank, die grind and heat & air blast methods. I haven't burned down anything or sliced or ground through anything yet. They all require care and patience. But the discussion so far has dealt with removing tubes. We need to discuss the difficulty of replacing the tube. ---We are talking about lugged frames here, I presume.--- If it's a top tube, the downtube will flex enough without yeilding to allow the headtube to move forward to slip a new top tube into the respective lugs and then letting it "spring" back into place... This is the easiest tube of the three main tubes and the most commonly damaged tube that needs replacing. If it's collision damage then most likely both the top tube and down tube need replacing. In that case, the head tube is removed and the top and down tubes are placed into the seat lug and bottom bracket and then the head tube is replaced as a whole. If it's the seat tube that needs replacing, then it will have to go down through the seat lug and into the bottom bracket as there is no ability to flex anything out of the way to insert the tube if the top, down, seat and chain stays are still intact. This presents it's own challenges and in my estimation is the most difficult tube to replace. The bottom bracket is the largest lug with the most connections so it is the most difficult to heat up for removing one tube end without disturbing the other tubes or inducing so much heat into it that it causes weakening of the other tubes.
I won't expound any further as I think I've made my point that it's not the removal of the damaged tube(s) that presents the most difficulty, it's replacing the new tube(s)that's challenging.
Bruce Schrader San Francisco, CA
===== "Not all those that wander are lost." -J.R.R. Tolkien (1892-1973)