A coolant subsystem for use in a fuel processor and a method for its operation are disclosed. In accordance with a first aspect, the coolant subsystem is separate from the feed to the processor reactor and is capable of circulating a coolant through the processor reactor. In accordance with a second aspect, the constituent elements of the fuel processor are housed in a cabinet, and the coolant subsystem is capable of cooling both the processor reactor and the interior of the cabinet. In various alternatives, the fuel processor can be employed to reform a fuel for a fuel cell power plant and/or may be used to provide thermal control for unrelated mechanical systems.