Direct numerical simulations (DNS) of combustion in globally lean non-homogeneous propane-air mixtures have been performed for several initial distributions of non-homogeneities. Results show the strong influence of the PDF and length scales of this distribution on both the combustion efficiency and thermal NO production, in qualitative accordance with experimental results. The simulations have been used to quantitatively assess flamelet models for heat release in partially premixed combustion and NO production modeling assumptions; good agreement is found for the models tested. A qualitative estimation of flamelet models for the secondary reaction zone has shown a limited range of application for those models, as only some of the intervening reactions were found to present a stationary structure in mixture fraction space. A conditional moment closure (CMC) model for the secondary reaction zone appears to be more satisfactory.