A reactor system for gas phase reacting of at least two fluid feed streams, where the reactor system has an injectively-mixed backmixing reaction chamber in fluid communication with a tubular-flow reactor. The injectively-mixed backmixing reaction chamber has a bulkhead that slides during real-time operation to either diminish or expand the internal volume of the backmixing reaction chamber. In one embodiment, the effective passageway space through the bulkhead is also variably adjustable. In another embodiment, the tubular-flow reactor shares the bulkhead so that axial bulkhead movement commensurately expands one reaction space while diminishing the other reaction space. Input gas streams enter the backmixing reaction chamber with sufficient velocity to turbulently agitate the contents of the injectively-mixed backmixing reaction chamber by injective intermixing of the alkane-containing gas feed stream and the oxygen-containing gas feed stream. A focal application is for direct (partial) oxidative conversion of natural gas to alkyl oxygenates.