Hybrid thermochemical processes show promise to increase plant performance with respect to fungible hydrocarbon production as a substitute to petroleum-based transportation fuels. Biomass, methane, and steam were reacted in a high temperature, indirectly heated reactor to determine the effects of biomass type (microalgae, rice hulls, cotton stalk), temperature (1600-1800 K), and reactant ratios (alpha = 0-2.0; beta = 1.0-4.8) on carbon conversion, cold gas efficiency, and syngas composition. This hybrid co-feed system was shown to achieve high H-2-content syngas with CO selectivity >0.90 and carbon conversion of both biomass and methane >0.90. Temperature was the dominant factor on the yields of CO, CO2, and CH4, while reactant ratios could be used to fine-tune the syngas composition. H-2 yield was only slightly dependent on temperature and excess steam. CO formation was highly kinetically-limited for this temperature range. Biomass type slightly affected gasifier performance, most likely due to total char and soot yield from devolatilization. Allothermal reactor design results in comparable gasifier efficiencies depending on steam input and thermal efficiency; a solarthermal reactor would negate 1.3-1.6 kgCO(2)/kgC processed and represents the recommended configuration for this type of process operation. (C) 2015 Elsevier Ltd. All rights reserved.