A four-tube heating system for combusting a fuel and transferring the heat released therefrom to a process fluid. The heating system includes, a fuel introduction zone, a combustion zone, an oxidant introduction zone, and a process fluid zone, wherein the fuel introduction zone is defined by fuel introduction means for introducing fuel into the combustion zone that is defined by a reaction tube external to and surrounding the fuel introduction means, and wherein the oxidant introduction zone is defined by an oxidant introduction tube external to and surrounding the reaction tube, and wherein the process fluid zone is defined by a process tube external to and surrounding the oxidant tube.