The machinery and methods disclosed herein are based on the use of a specialized extruder configured to continuously convey and plasticize/moltenize selected lignocellulosic biomass and/or waste plastic materials into a novel variable volume tubular reactor, wherein the plasticized/moltenized material undergoes reaction with circumferentially injected supercritical water—thereby yielding valuable simple sugar solutions and/or liquid hydrocarbon mixtures (e.g., “neodiesel”), both of which are key chemical commodity products. The reaction time may be adjusted by changing the reactor volume. The machinery includes four zones: (1) a feedstock conveyance and plasticization/moltenization zone; (2) a steam generation and manifold distribution zone; (3) a central supercritical water reaction zone; and (4) a pressure let-down and reaction product separation zone. The machinery and methods minimize water usage—thereby enabling the economic utilization of abundant biomass and waste plastics as viable renewable feedstocks for subsequent conversion into alternative liquid transportation fuels and valuable green-chemical products.