How to remove a cast iron pipe plug that has been soldered with lead
The pipes were sealed with lead. Prior to pouring the lead a packing material ("oakum" not sure on spelling) was packed into the joint. This material can be cut out with a screwdriver, trying to heat it would make a big mess. In most cases when I need to open a cast drain I will cut a small section out at least the diameter of the pipe but usually slightly longer. Once the section is out the pipe can wiggle and this makes removing the lead oakum seal a bit easier. I usually repair with fernco / no hubs. 

The one caution cast is heavy and you have a verticle run, make sure it is supported prior to cutting or it may separate in the wall at the next joint or pull down on a toilet flange and cause a leak there. The one positive thing if you support the cast and cut in these pipes don't leak as often because the water is not sitting in the pipes.