It is a 1997 size 18. The seat post isn't all that long, so it can't be very far into the frame. Those posts were prone to bending and once they bend, they can easily jam into the seat tube. I wouldn't heat it under any circumstances. If the worst comes to the worst, the post can be cut out of the tube. But first I would clamp the post onto a bench extremely firmly and try to turn the frame relative to the post, holding the furthest extremities of the frame for maximum leverage (keep a hub or wheel in to avoid any danger of distorting the stays).