A comprehensive multi-scale modeling approach is described to predict the dynamic evolution of the particle size distribution (PSD), molecular weight distribution (MWD), and rheological properties of the polyolefin particles produced in a catalytic Ziegler-Natta gas-phase olefin polymerization reactor. The overall MWD of a single polymer particle is calculated by the weighted sum of the MWDs of all polyolefin fractions produced over the different active sites of the multi-site catalyst. Numerical simulations are carried out, using the multiscale model, to investigate the effect of particle agglomeration on the distributed molecular polymer properties of the polyolefin. It is shown that agglomerated particles of less than 600 Am in diameter can exhibit different distributed molecular properties (i.e., MWD, CCD) as compared to those of the non-agglomerated particles of the same size. It is also shown that the agglomerated particles can exhibit bimodal MWDs even though the MWDs of the individual non-agglomerated particles are unimodal. Finally, the effect of particle agglomeration on representative rheological properties of the polyolefin ( e. g., shear viscosity and melt flow index, MFI) is investigated. It is demonstrated that agglomerated particles with particle diameters less than 600 Am can display lower shear viscosities and higher MFI values as compared to those of the non-agglomerated particles of the same size. Moreover, it is revealed that for polymer particles larger than 600 Am in diameter the respective particle shear viscosity and MFI values become almost independent of the particle size.