Computational fluid dynamics (CFD) simulations are used to simulate stacks of different sizes, to understand nonlinear effects that arise in scaleout of microchemical systems. As an example process, syngas production from methane is studied using a multifunctional, parallel-plate reactor with alternating combustion and steam reforming channels. A scale-out strategy is proposed that creates larger stacks from a base unit. Stacks are evaluated in terms of efficiency, maximum wall temperature, and stability under external heat loss for both high and moderate wall thermal conductivities. We find that smaller stacks are unstable under laboratory heat-loss conditions. Stacks with high wall thermal conductivities are more stable than those with moderate wall conductivities under our conditions. At high heat-loss coefficients, significant transverse thermal gradients exist between interior and edge channels of the stacks that result in a significant loss of efficiency. A transition from all ignited to some ignited and extinguished and finally to all extinguished channels is discovered as criticality is approached in moderate size stacks. Microsystems provide 1-3 orders of magnitude larger volumetric and gravimetric throughputs than conventional technology, irrespective of model uncertainty, and such intensification is central to portable and distributed processing. They exhibit energy efficiency that is a strong function of size and heat loss but can outperform conventional processing under many conditions. However, they result in higher cost per unit syngas volume unless system optimization is performed.