This work investigates an intensified membrane reactor with a monolithic square-channel configuration using three-dimensional computational fluid dynamics (CFD), with particular emphasis on steam methane reforming on the permeate side coupled with hydrogen–air combustion on the retentate side under a laminar diffusive flame regime. The proposed configuration enables direct thermal interaction between the reforming and combustion zones, allowing autothermal operation while maintaining a compact reactor layout. Hydrogen selectively permeates through the membrane, influencing local species distributions on the permeate side and enhancing reforming activity, while the permeated hydrogen sustains diffusive combustion on the retentate side without the need for premixing. The CFD model resolves the coupled behaviour of fluid flow, heat transfer, species transport, chemical reactions, and membrane permeation, and includes detailed SMR kinetics, a reduced H₂–air combustion mechanism suitable for laminar diffusion flames, and a NOₓ formation sub-model to account for emission characteristics. Radiative heat transfer is incorporated to capture the high-temperature thermal interactions within the monolithic structure. A systematic parametric study is carried out to examine the influence of reactor length and key operating parameters, including inlet temperature, operating pressure, steam-to-carbon ratio, flow configuration (co-current and counter-current), and combustion-side equivalence ratio. The analysis focuses on hydrogen production, methane conversion, membrane hydrogen flux, temperature fields, flame structure, and NOₓ emissions. Based on the obtained results, optimal operating ranges are identified that ensure stable diffusive combustion, improved reforming performance, and controlled emission levels. The findings provide practical insight into the coupled thermal and chemical behaviour of membrane-assisted reforming systems with diffusive combustion and offer design and operating guidance for compact, autothermal membrane reactors relevant to hydrogen production and integrated energy applications.