Exchanger-reactor comprising at least three stages with at least one stage comprising both: - millimetric channels at least partially covered by a catalyst capable of promoting a steam-reforming reaction, and - millimetric channels at least partially covered with a catalyst capable of promoting a water-gas reaction, characterised in that said exchanger-reactor comprises the following three stages superimposed: - at least one first stage comprising a first zone of millimetric channels making it possible for the circulation of a fluid at a temperature greater than 700°C to provide part of the heat necessary for a steam-reforming reaction, and a second zone of millimetric channels making it possible for the circulation of a fluid at a temperature less than 650°C and preferably less than 500°C to recover the heat generated by a water-gas reaction; - at least one second stage comprising a first zone of millimetric channels at least partially covered with a catalyst capable of promoting a steam-reforming reaction and a second zone of millimetric channels at least partially covered with a least one catalyst capable of promoting a water-gas reaction; and - at least one third stage comprising a first zone of millimetric channels making it possible for the circulation of the synthesis gas obtained by the steam-reforming reaction to recover part of the heat necessary for the steam-reforming reaction, and a second zone of millimetric channels making it possible for the circulation of a fluid at a temperature less than 650°C and preferentially less than 500°C to recover the heat generated by the water-gas reaction, the third stage being framed by two second stages and the assembly thus formed by the third stage and the two second stages is itself framed by two first stages. - millimetric channels at least partially covered by a catalyst capable of promoting a steam-reforming reaction, and - millimetric channels at least partially covered with a catalyst capable of promoting a water-gas reaction, - at least one first stage comprising a first zone of millimetric channels making it possible for the circulation of a fluid at a temperature greater than 700°C to provide part of the heat necessary for a steam-reforming reaction, and a second zone of millimetric channels making it possible for the circulation of a fluid at a temperature less than 650°C and preferably less than 500°C to recover the heat generated by a water-gas reaction; - at least one second stage comprising a first zone of millimetric channels at least partially covered with a catalyst capable of promoting a steam-reforming reaction and a second zone of millimetric channels at least partially covered with a least one catalyst capable of promoting a water-gas reaction; and - at least one third stage comprising a first zone of millimetric channels making it possible for the circulation of the synthesis gas obtained by the steam-reforming reaction to recover part of the heat necessary for the steam-reforming reaction, and a second zone of millimetric channels making it possible for the circulation of a fluid at a temperature less than 650°C and preferentially less than 500°C to recover the heat generated by the water-gas reaction,