Unsustainable dependence on fossil fuel reserves for energy and material demands is leading to growing amounts of CO2 concentration in the atmosphere and irreversible climate changes. Carbon neutral sources such as abundant biomass reserves and landfill-destined high energy density wastes such as plastics, and tire-wastes can be utilized together for energy and material production for a sustainable future. Pyrolysis and gasification can convert these variable feedstocks into valuable and uniform synthetic gas (syngas) with versatile downstream applicability to energy, liquid fuels, and other value-added chemicals production. But seasonal availability, high moisture and ash content, and relatively low energy density of biomass can result in significant energy and economic losses during gasification. Furthermore, gasification of plastic wastes separately was found to result in feeding issues due to melt-phase, coking, and agglomerative behavior leading to operational issues. To resolve these issues, co-processing of biomass with these plastics and rubber wastes was found to be promising in addition to providing synergistic interaction leading to enhanced syngas yield and inhibitive behavior in some cases and thus motivating this work. This dissertation provides a deconvoluted understanding and quantification of the source and impact of these interactions for better process performance and alleviation of inhibitive interaction needed to develop reliable co-gasification of feedstock mixtures. To achieve this, plastic and tire wastes were investigated separately and mixed with different biomass species using a series of feedstock arrangements to understand synergistic influence on the syngas yield and kinetics in comparison to mono-conversion. Influence of operating conditions such as feedstock composition, temperature and gasifying agent was also examined for desirable conditions of energy recovery and high-quality syngas yield. Lab-scale semi-batch reactor studies equipped with online product gas analysis, along with thermogravimetric studies were utilized to obtain insight into the products yield, kinetics, and energy conversion. These results provided a better understanding of the influence of feedstocks and their interaction on the syngas and process behavior. They address the knowledge gap in versatile feedstock-flexible gasifier development for efficient and reliable syngas production from varying solid waste and biomass component mixtures with minimal changes to the operating conditions.