A reactor for reacting gaseous or vaporized hydrocarbons with water vapor to produce carbon monoxide and hydrogen containing gases in the presence of a catalyst. The catalyst is contained in indirectly heated tubes in a heating zone which contains the gaseous heating fluid. The catalyst tubes extend through the reactor housing and at least partly into a heating zone defined by the reactor housing. The portion of the tubes extending into the reactor housing are surrounded by an inner shielding wall which is permeable to an outer shielding wall disposed within the reactor housing and defining a flow passage therewith.