A reactor in the form of an essentially horizontal cylinder for carrying out an autothermal gas-phase dehydrogenation of a hydrocarbon-comprising gas stream by means of an oxygen-comprising gas stream to give a reaction gas mixture over a heterogeneous catalyst configured as monolith is disclosed herein. The interior of the reactor is divided by a detachable, cylindrical or prismatic housing which is arranged in the longitudinal direction of the reactor, gastight in the circumferential direction and open at two end faces of the housing into an inner region, having one or more catalytically active zones. In each case, a packing composed of monoliths may be stacked on top of one another, next to one another, and above one another and before each catalytically active zone. Also in each case, a mixing zone having solid internals and an outer region arranged coaxially to the inner region are provided.