Various embodiments of the present invention are directed to multi-step systems and methods for target-molecule purification that employ column-chromatography-based and/or membrane-filtration-based polishing steps. In one described embodiment of the present invention, a target-protein-containing eluate having a high residual salt concentration is collected from a first chromatography column prepared with an affinity-chromatography resin, loaded onto a second chromatography column prepared with a cation-exchange resin, and eluted from the second cation-exchange column using a buffer in which a time-dependent pH gradient is established. In another described embodiment of the present invention, a partially purified target-protein-containing eluate is collected from a chromatography column and further purified by passing the target-protein-containing eluate through a salt-tolerant anion exchanger.