Hi Tarah
I presume to plan on tiling with ceramic tiles? To do it the best possible way is to have the floor professionally ground which tales a milimetre or two off the concrete and leaves no trace of the old glue at all. This is my preferred method, however there is a product out that you can get from the tile shops that you mix up and spread over the old glue that bonds to it and you can then tile as normal over it. A very good product that for the life of me I cannot recall it's name. I have never heard of using a heat gun and not sure that it would work too well.