NOVELTY - A thermally-integrated hydrogen production unit (1) is started-up by, supplying a start-up burner (3) with mixture of hydrocarbon feedstock and air to produce a gaseous stream (200 degrees C); evaporating a stream of water by indirect heat exchange with hot stream(s), generating a stream of water vapor (300 degrees C); evaporating hydrocarbon feedstock if liquid or heating if gaseous, by indirect heat exchange with hot stream(s); introducing feedstock into reforming reactor (2); and shutting down start-up burner and lighting permanent operating burner (4) that uses residual air and gas.    USE - Method for starting-up thermally-integrated hydrogen production unit (claimed), useful for producing highly thermally-integrated hydrogen used as alternative fuel, i.e. fuel cells.    ADVANTAGE - The method produces highly thermally-integrated hydrogen by reforming a hydrocarbon feedstock.    DETAILED DESCRIPTION - Starting-up a thermally-integrated hydrogen production unit (1) comprises:    (A) supplying a burner (3), which ensures start-up of unit, with a mixture of hydrocarbon feedstock and air to be burned to give hot effluents that heat a gaseous stream that enters and exits a reforming reactor, until the temperature of the stream that is obtained from the reforming reactor reaches more than 200 degrees C;    (B) evaporating a stream of water by indirect heat exchange with hot stream(s) of the process, generating a stream of water vapor that enters the reforming reactor until the stream that is obtained from the reactor reaches more than 300 degrees C and all the water is evaporated;    (C) evaporating the hydrocarbon feedstock to be reformed when the feedstock is a liquid, or heating when the feedstock is gaseous, by indirect heat exchange with hot stream(s) of the process;    (D) introducing the feedstock into the reforming reactor (2) to produce a hydrogen-rich effluent stream; and    (E) shutting down the start-up burner, lighting a burner (4) that is in permanent operating using residual air and gas discharged by a section for purifying the hydrocarbon stream as fuel.    The hydrocarbon feedstock is preferably liquid. Stabilized operating conditions are maintained between 5 and 15 minutes. There are less than 30 seconds of waiting time between the shutdown of the start-up burner and the lighting of the residual gas burner so as to prevent the cooling of the entire hydrogen production unit. The feedstock stream is preferably regulated so that the flow rate is 15-50% of the flow rate that is imposed in permanent operation, and this circulation of stream is maintained until the temperature at the outlet of the reformer is preferably more than 300 degrees C. The waiting period is used for carrying out a flushing of air or an inert gas in the start-up burner.    DESCRIPTION OF DRAWING(S) - The drawing is a process diagram that shows an installation variant for reforming a hydrocarbon feedstock.    Hydrogen production unit (1)    Reforming reactor (2)    Start-up burner (3)    Permanent operating burner (4)    Purification section (5)    Heat exchangers (6, 7)    Air compressors (15, 19, 23)