The present invention relates to a process and a device for manufacturing synthesis gas. The reactor according to the invention comprises within a single housing: A non catalytic combustion chamber (1) comprising at least one device (2) for injecting a fuel and at least one device (3) for injecting an oxidizer so as to achieve a partial combustion in said chamber, and at least one catalytic bed (4) which the gases coming from combustion chamber (1) run into, and also comprising at least one device (6) for injecting an additional oxidizer. The reactor and the process according to the invention may be applied to any chemical manufacturing utilizing synthesis gas, such as the manufacturing of ammonia, urea, methanol, higher hydrocarbons, etc.