A polymerization reactor system includes features for operating two or more reactors in series or parallel. The system includes a discharge line coupled to an outlet of a first polymerization reactor, a first conduit connecting the first polymerization reactor to a second polymerization reactor via the discharge line, a second conduit connecting the first polymerization reactor to a flashline apparatus via the discharge line, and a diverter valve coupling the first and second conduits to the discharge line. The diverter valve is configured to selectively open the discharge line to the first conduit and close the discharge line to the second conduit in a first valve state and to selectively open the discharge line to the second conduit and close the discharge line to the first conduit in a second valve state. The diverter valve is capable of switching between the first and second valve states.