This study comprehensively reviews biomass gasification and its empirical correlations for syngas production. The types of biomass, reactors, gasification agents, and gasification operating conditions were evaluated thoroughly using PRISMA and meta-analysis approach. Decreasing syngas production was observed in air gasification with increasing ER, but increasing syngas was observed in steam gasification with increasing S/B, both at a constant temperature. Biomass properties dictate syngas production in air gasification, whereas the optimum syngas production is effectively driven by the S/B value. The predicted total syngas yield may have the global maximum, the global minimum and the saddle-point optimisation form, depending on the gasification agents and the reactor types. Air gasification of MSW and wood pellets, and steam gasification of wood pellets might be performed using a fluidised-bed or updraft reactor because their mean syngas productions are not significantly different. Steam gasification with sawdust resulted in significantly different syngas production in downdraft, updraft, and fluidised-bed reactors. This study is concluded by providing guidelines for selecting the most appropriate biomass for gasification, the required operating parameters according to the type of gasification agent and the gasifier reactor.