A CO preferential oxidation reactor comprising: • a first heat exchanger for partially cooling a gas stream with a high H 2 concentration by means of a water stream that is transformed into a steam stream, followed by a first CO preferential oxidation reactor module having a catalyst bed, for carrying out the partial purification of the gas stream with a high H 2 concentration by feeding a pure O 2 stream that is injected by feeding means in the inlet pipe of the gas stream with a high H2 concentration of the first heat exchanger, • a second heat exchanger disposed at the outlet of the first preferential oxidation reactor module for cooling the gas stream with a high H 2 concentration by means of a water stream that is transformed into a steam stream, followed by a second CO preferential oxidation reactor module having a catalyst bed, wherein the partial purification of the gas stream with a high H 2 concentration continues by feeding a pure O 2 stream that is injected by feeding means at the outlet of the first preferential oxidation reactor module, prior to the inlet of the second heat exchanger, and • a third heat exchanger disposed at the outlet of the second preferential oxidation reactor module for cooling the gas stream with a high H 2 concentration by means of the water stream that is transformed into a steam stream, followed by a third preferential oxidation reactor module having a catalyst bed, wherein the partial purification of the gas stream with a high H 2 concentration continues by feeding a pure O 2 stream that is injected by feeding means at the outlet of the second preferential oxidation reactor module, prior to the inlet of the third heat exchanger, characterised in that the total number of tubes of the three heat exchangers (1, 10, 12) is integrated in a single shell, wherein the shell is cylindrical and the tubes are disposed longitudinally inside said shell, wherein an inlet manifold that feeds the reformate gas to the heat exchange and flow distribution tubes is disposed on one of the bases of the cylindrical shell, while an outlet manifold that collects the reformate gas that flows out of the tubes and feeds it to each of the CO preferential oxidation reactor modules (5, 11, 13) is disposed on the other base of the cylindrical shell. • a first heat exchanger for partially cooling a gas stream with a high H 2 concentration by means of a water stream that is transformed into a steam stream, followed by a first CO preferential oxidation reactor module having a catalyst bed, for carrying out the partial purification of the gas stream with a high H 2 concentration by feeding a pure O 2 stream that is injected by feeding means in the inlet pipe of the gas stream with a high H2 concentration of the first heat exchanger, • a second heat exchanger disposed at the outlet of the first preferential oxidation reactor module for cooling the gas stream with a high H 2 concentration by means of a water stream that is transformed into a steam stream, followed by a second CO preferential oxidation reactor module having a catalyst bed, wherein the partial purification of the gas stream with a high H 2 concentration continues by feeding a pure O 2 stream that is injected by feeding means at the outlet of the first preferential oxidation reactor module, prior to the inlet of the second heat exchanger, and • a third heat exchanger disposed at the outlet of the second preferential oxidation reactor module for cooling the gas stream with a high H 2 concentration by means of the water stream that is transformed into a steam stream, followed by a third preferential oxidation reactor module having a catalyst bed, wherein the partial purification of the gas stream with a high H 2 concentration continues by feeding a pure O 2 stream that is injected by feeding means at the outlet of the second preferential oxidation reactor module, prior to the inlet of the third heat exchanger,