Generator for producing an endothermic gas mixture by means of a reaction of incomplete combustion of a gaseous mixture of air and methane or another hydrocarbon, in stoichiometric defect of oxygen and without water/steam, said generator comprising - an outer cylindrical heating chamber that supplies heat to a catalytic reactor contained inside said outer chamber, said reactor containing a noble metal oxide-based catalyst for the partial oxidation of a mixture of air-methane or another hydrocarbon; - at least three catalyst tubes, parallel one to the other, arranged in the form of a tube bundle inside said catalytic reactor, each of said tubes having a nominal diameter equal to or smaller than 80 mm, - at least one gas/air heat exchanger for the cooling of the produced endothermic gas mixture, said heat exchanger being in the form of a duct and placed outside of said outer chamber, said exchanger comprising at least one cooling tube, each cooling tube being in fluid communication with a respective catalyst tube, said generator being characterised in that said catalytic reactor being constituted of an inner chamber containing inside said catalyst tubes, each catalyst tube is in fluid communication with a respective supply tube, external to said heating chamber, for supplying said catalyst tube with a flow (Qi) which is a portion of a predetermined nominal flow rate (Q N) of said mixture of air and methane or another hydrocarbon, each catalyst tube containing the catalyst only in one section with respect to the length of the catalyst tube contained in the heating chamber, so that the remaining empty section constitutes a heating chamber of the mixture of air and methane or another hydrocarbon, said each supply tube being provided with a respective shut-off valve, said shut-off valves being adapted to exclude singly said supply flow (Qi) of one or more of said supply tubes so as to obtain a smaller overall supply flow with respect to said nominal flow (Q N), said overall flow being the sum of the flows (Qi) in input to each supply tube in operation; and characterized in that said outer cylindrical chamber comprises internally a plurality of burners heating said inner chamber and placed in an opposite manner with respect to said inner cylindrical chamber and aligned vertically in such a way as to heat uniformly the entire outer surface of said inner chamber containing said catalyst tubes. - an outer cylindrical heating chamber that supplies heat to a catalytic reactor contained inside said outer chamber, said reactor containing a noble metal oxide-based catalyst for the partial oxidation of a mixture of air-methane or another hydrocarbon; - at least three catalyst tubes, parallel one to the other, arranged in the form of a tube bundle inside said catalytic reactor, each of said tubes having a nominal diameter equal to or smaller than 80 mm, - at least one gas/air heat exchanger for the cooling of the produced endothermic gas mixture, said heat exchanger being in the form of a duct and placed outside of said outer chamber, said exchanger comprising at least one cooling tube, each cooling tube being in fluid communication with a respective catalyst tube,