The invention relates to a process for the catalytic partial oxidation of a hydrocarbonaceous feedstock, wherein a feed mixture having the hydrocarbonaceous feedstock and a molecular-oxygen containing gas is contacted with a catalyst system having an upstream part and a downstream part, the downstream part being in the form of a porous catalyst bed, wherein the catalyst system is retained in a reactor, the reactor having an upstream part that contains the upstream part of the catalyst system and a downstream part that contains the downstream part of the catalyst system, wherein the upstream part of the catalyst system only partly fills the cross-sectional area of the fluid flow path of the upstream part of the reactor and the downstream part of the catalyst system completely fills the cross-sectional area of the fluid flow path of the downstream part of the reactor. The invention further relates to a reactor having such a catalyst system and a catalytic reaction zone for the water-gas shift conversion of the effluent of the catalyst system, to a fuel cell system having such a reactor and a fuel cell, and to a vehicle provided with such a fuel cell system.