A fuel conversion system having an exhaust gas generator, such as an engine or a furnace, having a heated exhaust gas outlet, a reactor vessel having a heated exhaust gas inlet in fluid communication with the heated exhaust gas outlet, having a cooled exhaust gas outlet, and containing at least one reactor tube having an exhaust gas inlet in fluid communication with the heated exhaust gas outlet, a convertible fuel inlet, and a converted fuel outlet, and a monolithic fuel conversion catalyst structure substantially centered within the at least one reactor tube and shaped to form at least one flow channel between the monolithic fuel conversion catalyst structure and an inside wall of the at least one reactor tube.