A continuous process for producing high impact polystyrene comprising feeding at least one vinyl aromatic monomer, an elastomer, and a free radical initiator to a first linear flow reactor to form a reaction mixture, polymerizing the reaction mixture in said linear flow reactor to at least the phase inversion point of the mixture, and feeding the reaction mixture from the first linear flow reactor to a second reactor for post-inversion polymerization of the mixture. A method of producing an elastomer-reinforced polymer comprising inverting a reaction mixture comprising at least one vinyl aromatic monomer, an elastomer, and a free radical initiator in a plug flow reactor. A high impact polystyrene reactor system, comprising a linear flow reactor having an inlet and an outlet, and a continuously stirred tank reactor having an inlet in fluid communication with the linear flow reactor outlet and receiving an effluent from the linear flow reactor.