NOVELTY - A reactor system (1200) for gas phase reacting of at least two fluid feed streams into a product stream comprises: an injectively-mixed backmixing reaction chamber (1202) in fluid communication with a tubular-flow reactor (1204) and having a backmixing reaction chamber housing (1206); and a bulkhead (1232) in slideably-sealed interface to the backmixing reaction chamber housing. The backmixing reaction chamber has an injectively-mixed backmixing reaction chamber internal volume (1234) defined by the housing and by the bulkhead.    USE - For gas phase reacting of at least two fluid-feed streams into a product stream i.e. for direct (partial) oxidative conversion of natural gas (e.g. methane, ethane, propane, or butane) to alkyl oxygenates e.g. methanol, ethanol and formaldehyde (claimed).    ADVANTAGE - The reactor system enables individualized control of a primary free-radical induction sub-reaction separately from subsequent sub-reactions that respond to the induced free radicals and is efficient and low cost.    DETAILED DESCRIPTION - A reactor system (1200) for gas phase reacting of at least two fluid feed streams into a product stream, comprises: an injectively-mixed backmixing reaction chamber (1202) in fluid communication with a tubular-flow reactor (1204) and having a backmixing reaction chamber housing (1206); and a bulkhead (1232) in slideably-sealed interface to the backmixing reaction chamber housing. The backmixing reaction chamber has an injectively-mixed backmixing reaction chamber internal volume (1234) defined by the housing and by the bulkhead. The housing has a housing portion in opposite disposition to the bulkhead. The bulkhead is slideably movable during real-time operation of the reactor system to progress within the backmixing reaction chamber housing toward the housing portion to commensurately diminish the injectively-mixed backmixing reaction chamber internal volume. The bulkhead is alternatively slideably movable during real-time operation of the reactor system within the backmixing reaction chamber housing to retract away from the housing portion to commensurately expand the injectively-mixed backmixing reaction chamber internal volume.    DESCRIPTION OF DRAWING(S) - The figure shows a cross section simplified view of the reactor system having an injectively-mixed backmixing reaction chamber in close coupling to a tubular-flow reactor.    a reactor system (1200)    an injectively-mixed back-mixing reaction chamber (1202)    tubular flow reactor (1204)    a backmixing reaction chamber housing (1206)    bulkhead (1232)    internal volume. (1234)