Steam gasification of combined biomass and plastics at different mass ratios of biomass to plastic was investigated in a semi-batch reactor at 1173 K and atmospheric pressure using different types of plastics. The specific plastics examined were black polycarbonate (BPC), polyethylene-terephthalate (PET), and polypropylene (PP). The chemical composition of the syngas yield evolved measured using a microGC revealed synergistic effects on the role of biomass and plastics during gasification. The observed synergy was quantified from a direct comparison of the results on cumulative gas yields from biomass-plastics mixtures as compared to the corresponding weighted aggregate results from the gasification of separate feedstock components. The results showed that the total syngas as well as H-2, CO, and CO2 yields enhanced using mixtures, while the light hydrocarbon gas yields reduced in the order of PP > BPC > PET. The increase of H2 and CO with the reduction in hydrocarbon yield revealed synergistic enhancements from the secondary steam reforming reactions. TGA data from co-pyrolysis of BPC and PET with biomass revealed significant interaction while no such interaction was observed with PP. The carbon conversion and energy efficiency results showed enhancement for the BPC case. Increase in product gas yield suggests greater reformation of biomass plastic mixtures during gasification. Further studies are required to understand fundamental reaction mechanisms as well as demonstrative gasifier studies to understand both the kinetics and practical feasibility.