NOVELTY - Transforming a selected polymeric material into reaction products involves conveying the selected material through an extruder so as to define selected polymeric material flowstream; injecting hot compressed water into the supercritical fluid reaction zone while the extruder is conveying the flowstream into reaction zone to form a mixture; retaining the mixture within reaction zone for time period to yield reaction products, where reaction zone is a tubular reactor having inner tubular spear; and expelling reaction products out of the reaction zone and into reaction products chamber.    USE - For transforming a selected polymeric material (i.e. biomass and/or waste plastic) into reaction products (claimed), particularly simple sugars, hydrocarbons, and/or aromatic chemicals which are readily converted into liquid transportation fuels.    ADVANTAGE - The use of spear-and-tube reactor in the present method allows a controlled and/or minimal amount of supercritical water to enter into the system (i.e. preferably less than 100 wt.%).    DETAILED DESCRIPTION - Transforming a selected polymeric material into reaction products involves: conveying the selected polymeric material through an extruder so as to define a selected polymeric material flowstream, where the extruder is configured to continuously convey the selected material from an upstream inlet to a supercritical fluid reaction zone; injecting hot compressed water into the supercritical fluid reaction zone while the extruder is conveying the selected material flowstream into the supercritical fluid reaction zone so as to yield a mixture; retaining the mixture within the reaction zone for a time period to yield reaction products, where the reaction zone is a tubular reactor having an inner tubular spear, and the tubular reactor and the inner tubular spear further define an annular space within the reaction zone, and the mixture flows through the annular space; and expelling the reaction products out of the supercritical fluid reaction zone and into a reaction products chamber.