Multiple pyrolytic interactions in the cellulose-hemicellulose-lignin-polyethylene system were thoroughly investigated using a novel approach, which evaluates the pyrolytic interaction impacts through product recovery tests and the in-situ pyrolysate monitoring by evolved gas analysis-mass spectrometry. Fast pyrolysis of neat cellulose, xylan, milled wood lignin, beech wood, polyethylene, and their mixtures was conducted at 650 °C using the combined approach. Interactions in the cellulose/polyethylene, xylan/polyethylene, and milled wood lignin/polyethylene systems enhanced the production of CO and C₂-C₃ hydrocarbons, and simultaneously inhibited that of solids such as heavy tar, wax, and coke. Polyethylene worked as a dispersant of biomass upon injection, and the improved gasification was mainly due to the enhanced hydrogen-exchange between hydrogen-rich polyethylene pyrolysates and carbon-centered radicals in vapor phase. The results suggested that pyrolytic interactions in the biomass components (cellulose-hemicellulose-lignin) occur preferentially before contact with polyethylene, and then the beech wood pyrolysates further interact with the polyethylene pyrolysates. Thus, this novel approach allowed advanced evaluation of pyrolytic interactions and can be applied to different combinations and compositions in the cellulose-hemicellulose-lignin-plastic system. It will help understand the nature of co-pyrolysis systems and predict the significance of co-pyrolysis in practical energy and chemical feedstock production.