Reformer for converting a hydrocarbon fuel into hydrogen gas and carbon dioxide comprises: (a) a first vessel having a partial oxidation reaction zone and a separate steam reforming reaction zone, the first vessel having a first inlet at the partial oxidation zone and a first outlet at the steam reforming zone, (b) a helical tube extending about the first vessel which has a first end for connection to an oxygen source and a second end connected to the first vessel inlet for flow of oxygen gas, and (c) a second vessel having second vessel inlet and outlet, the second vessel being annularly disposed about the first vessel, the helical tube situated between the vessels so that the gases from the first vessel can be directed through the second vessel.    Also claimed is a method for converting a hydrocarbon fuel into hydrogen gas and carbon dioxide which involves: (a) directing an oxygen gas through the helical tube, (b) directing a hydrocarbon vapour and steam into the helical tube, (c) directing the mixture of hydrocarbon vapour, steam and oxygen gas into the first vessel so that the hydrocarbon vapour spontaneously partially oxidised to form a heated reformate stream which includes carbon monoxide and hydrogen gas, (d) steam-reforming the remaining hydrocarbon vapour in the heated reformate stream to form hydrogen gas and carbon monoxide, (e) directing the heated reformate stream over the helical tube, (f) converting part of the carbon monoxide gas of the reformate stream to carbon dioxide gas by a high-temperature shift reaction and (g) converting at least part of the remaining carbon monoxide gas of the reformate stream to carbon dioxide by a low-temperature shift reaction, thus forming a product stream including hydrogen gas and carbon dioxide.    USE -   For converting hydrocarbon fuel into hydrogen gas and carbon dioxide, especially in fuel cells for powering vehicles.    ADVANTAGE -   Compact and lightweight reformer which does not form soot and provides heat to the steam reforming zone.