A reactor includes an essentially horizontal cylinder for carrying out an autothermal gas-phase dehydrogenation of a hydrocarbon-comprising gas stream using an oxygen-comprising gas stream to give a reaction gas mixture over a heterogeneous catalyst configured as monolith. The interior of the reactor is divided by a detachable, cylindrical or prismatic housing, which is arranged in the longitudinal direction of the reactor and is gastight in the circumferential direction, into an inner region having one or more catalytically active zones, each having a packing composed of monoliths stacked on top of one another, next to one another and behind one another and before each catalytically active zone in each case a mixing zone having solid internals are provided and into an outer region, which is supplied with an inert gas, arranged coaxially to the inner region. A heat exchanger is connected to the housing at one end of the reactor.