Multi-stage polymerization processes that produce polymers having controlled compositions and molecular weight with improved catalyst productivity are disclosed. An example method for producing a multi-modal polyolefin comprises polymerizing the multi-modal polyolefin in the presence of a metallocene catalyst system in at least a slurry-phase polymerization stage and a gas-phase polymerization stage arranged in series in any order, wherein the multi-modal olefin comprises: (i) a first polyolefin fraction having a density of less than about 940 kg/m3, and (ii) a second polyolefin fraction having a density of less than about 930 kg/m3.