A device (24; 124; 224) comprising: - an enclosure defining an interior volume and configured to contain a mixture of water and steam , - a first inlet (E1) suitable and intended for admitting water into the interior volume , and a first outlet (S1) suitable and intended for discharging steam from the interior volume , - a second inlet (E2) suitable and intended for receiving a syngas , and a second outlet (S2) suitable and intended for delivering a modified syngas , - a first exchanger-reactor (50A; 150A; 250A) located in the interior volume , fluidly connected to the second inlet (E2) and to the second outlet (S2), and suitable for carrying out a water-gas shift reaction of the syngas and obtaining the modified syngas , and for heating the water admitted into the interior volume and producing at least part of said steam , - a third inlet (E3) suitable and intended for receiving a combustible gas distinct from the syngas , a fourth inlet (E4) suitable and intended for receiving an oxidizing gas including at least 20% oxygen by volume, and a third outlet (S3) suitable and intended for discharging converted gases , and - a second exchanger-reactor (50B; 250B) fluidly connected to the third inlet (E3), to the fourth inlet (E4) and to the third outlet (S3), the second exchanger-reactor (50B; 250B) being suitable for carrying out a catalytic oxidation of the combustible gas by the oxidizing gas and obtaining the converted gases , characterized in that the second exchanger-reactor (50B; 250B) is located in the interior volume and is suitable for heating the water admitted into the interior volume and producing at least part of said steam . - an enclosure defining an interior volume and configured to contain a mixture of water and steam , - a first inlet (E1) suitable and intended for admitting water into the interior volume , and a first outlet (S1) suitable and intended for discharging steam from the interior volume , - a second inlet (E2) suitable and intended for receiving a syngas , and a second outlet (S2) suitable and intended for delivering a modified syngas , - a first exchanger-reactor (50A; 150A; 250A) located in the interior volume , fluidly connected to the second inlet (E2) and to the second outlet (S2), and suitable for carrying out a water-gas shift reaction of the syngas and obtaining the modified syngas , and for heating the water admitted into the interior volume and producing at least part of said steam , - a third inlet (E3) suitable and intended for receiving a combustible gas distinct from the syngas , a fourth inlet (E4) suitable and intended for receiving an oxidizing gas including at least 20% oxygen by volume, and a third outlet (S3) suitable and intended for discharging converted gases , and - a second exchanger-reactor (50B; 250B) fluidly connected to the third inlet (E3), to the fourth inlet (E4) and to the third outlet (S3), the second exchanger-reactor (50B; 250B) being suitable for carrying out a catalytic oxidation of the combustible gas by the oxidizing gas and obtaining the converted gases , characterized in that the second exchanger-reactor (50B; 250B) is located in the interior volume and is suitable for heating the water admitted into the interior volume and producing at least part of said steam . <EOS>