System comprising a reformer and a fuel cell and process for shutting down and starting up the same

Reformer (1) is operated in a first operational stage by the sub-stoichiometric reaction of hydrocarbons with oxygen, air and/or water to produce hydrogen, and in a second operational stage by the stoichiometric reaction of hydrocarbons with oxygen, air and/or water to produce inert gas or gas mixture. Independent claims are also included for the following: (1) Process for switching off a reformer-fuel cell system; and (2) Process for starting a reformer-fuel cell system.