Abstract Valorisation of plastic waste or forest-agroindustrial residues poses an opportunity to reduce landfilling and to obtain value-added products simultaneously. Through thermal partial oxidation, hydrocarbon fuels can be gasified and converted into syngas. Particularly, hybrid porous media reactors allow gasifying low-grade fuels in an efficient eco-friendly way. In this work, the natural gas-supported gasification of polyethylene and wood mixtures in a porous bed reactor is studied. The reactor is designed and built to operate in semi-continuous mode. The temperature evolution, the reaction wave propagation rate, and the resulting concentration of the main gas species (H2, CO, CH4, CO2, NOx, and UHC) were measured for different polyethylene and wood mixtures, keeping an optimum natural gas-to-air ratio of 0.8. The influence of the gasification conditions on the reaction pathways and the process development is analysed by sampling the system in two representative moments. The maximum syngas production was 23.86 vol%, which was obtained when loading only 100% polyethylene, while the maximum energy return on investment of 49% was registered for the highest biomass fraction in mixture. The semi-continuous operation design was validated based on the thermal behaviour and the combustion wave displacement. This experimental work shows promising results in the search for solutions to gasify solid wastes in continuous mode.