In the near future, biomass is expected to be a key resource for fulfilling clean energy requirements. The predictive modeling of biomass conversion in industrial systems is still challenging due to the multi-scale nature of the processes involved, including complex chemical reactions. To provide a detailed description and analysis of these reactions and their coupling to transport phenomena in large-scale systems, this work introduces a multiphase reactor network method incorporating efficient solution algorithms for engineering reacting systems. The method developed includes a novel multiphase solid–gas perfectly stirred reactor (sPSR) model capable of handling solid- and gas-phase reactions as well as heterogeneous reactions, in a fully coupled way. The sPSR approach, which is included in reactor networks with various levels of complexity, improves the analysis and understanding of physical and chemical phenomena occurring in gasifiers. Steam and air gasification experiments are analyzed using the new approach and fundamental differences and effects are revealed. Major findings include the importance of solid–gas coupling, char conversion reactions, and water–gas shift (WGS) reaction catalysis due to bio-ash. In addition, increasing the complexity of the reactor network achieves a better reproduction of the mixing behavior. The capability of handling complex reactor networks with detailed kinetics paves the way for efficient designs and improved reactors.