NOVELTY - The production of synthesis gas from feedstock, comprises adding first feedstock to water vapour for steam reforming in multitubular reactor-exchanger (2b) to form first synthesis gas, heating reaction tubes (38) by circulating a heating fluid in a shell to form second synthesis gas, and mixing the two gases. The feedstock comprises hydrocarbons and optionally recycled compounds. The multitubular reactor-exchanger comprises reaction tubes containing a steam reformer catalyst, and the shell.    USE - Useful to produce synthesis gas from feedstock. The synthesis gas is useful to produce hydrogen and liquid compounds by chemical conversion.    ADVANTAGE - The process is reliable, compact, coercive and cost effective.    DETAILED DESCRIPTION - The production of synthesis gas from feedstock, comprises adding first feedstock to water vapour for steam reforming in multitubular reactor-exchanger (2b) to form first synthesis gas, heating reaction tubes (38) by circulating a heating fluid in a shell to form second synthesis gas, and mixing the two gases. The feedstock comprises hydrocarbons and optionally recycled compounds. The multitubular reactor-exchanger comprises reaction tubes containing a steam reformer catalyst, and the shell. The reaction tube containing the heating fluid in which the fluid is heated by circulating the shell. The heating fluid is free from nitrogen and is partially oxidized with oxygen to form a second feedstock under a pressure of 0.5-12 Mpa. The partial oxidation complementary zone is placed in an inner side of the shell. The heating fluid comprises a first flow of partial oxidation of oxygen. The partial oxidation complementary zone is free from reaction tubes and contiguous internal wall of the shell. Inner side of the reactor-exchanger shell has a second feedstock and oxygen, which are injected into the shell in presence of water vapour in an upstream of a complementary partial catalytic oxidation zone under turbulence conditions or temperature of mixture is lower than the temperature or combustion occurs with the flame. The heating fluid is mixed with a second feedstock and oxygen in the partial oxidation complementary zone to increase the temperature of the first flow. The fluid is circulated to obtain heat in the shell before the fluid leaves from the reactor-exchanger. The flow comprises the first feedstock added to the water vapour that circulates in the reactor exchanger shell, and the catalytic combustion zone is placed at the end of the first half of the reactor-exchanger shell. The heating fluid is completely circulates through co-flow of the first feedstock. The partial oxidation complementary zone is arranged to the exterior of the reactor-exchanger shell. The two synthesis gases are mixed with the outside of the reactor-exchanger shell and the pressures of P1 and P2 at exit of the reactor-exchanger are 0.8 Mpa less than P1 less than 12 Mpa, 0.8 Mpa less than P2 less than 12 Mpa and modulus of P2-P1 is less than 0.35 Mpa. The heat is exchanged into the flow at the exit of the reactor-exchanger shell. The first feedstock is added to the water vapour for final pre-heating/partial steam reforming of the flow before the water vapour is entered into the reactor-exchanger. The second synthesis gas is mixed with an outgoing flow of the first synthesis gas to form a synthesis gas that is preheated in the exchanger. The flow comprises the first feedstock added to the water vapour, which is preheated between 500-700degreesC before the water vapour is entered into the reactor-exchanger shell. The temperature of the first and second synthesis gas in the exit of the reactor-exchanger are 800-950degreesC and 850-1150degreesC respectively. An INDEPENDENT CLAIM is included for a production of hydrogen from a synthesis gas.    DESCRIPTION OF DRAWING(S) - The diagram shows a device for producing synthesis gas:    Preheating exchanger; (2a)    Reactor-exchanger; (2b)    Shell; (37)    Reaction tubes; (38)    Partial oxidation chamber. (40)