A process for fluid phase in-line blending of plasticized polymers is provided. The process includes providing two or more reactor trains configured in parallel and a separator for product blending and product-feed separation, wherein one or more of the reactor trains produces one or more polymers and one or more of the reactor trains produces one or more plasticizers; contacting in at least one of the parallel reactor trains olefin monomers, catalyst systems, optional comonomers, optional scavengers, and optional diluents or solvents, at a temperature above the solid-fluid phase transition temperature of the polymerization system and a pressure no lower than 10 MPa below the cloud point pressure of the polymerization system and less than 1500 MPa; forming a reactor effluent including a homogeneous fluid phase polymer-monomer mixture and plasticizer-monomer mixture in each parallel reactor train; passing the reactor effluents through the separator; maintaining the temperature and pressure within the separator above the solid-fluid phase transition point but below the cloud point pressure and temperature to form a fluid-fluid two-phase system including a plasticized polymer-rich blend phase and a monomer-rich phase; and separating the monomer-rich phase from the plasticized polymer-rich blend phase. The polymer-rich blend phase is conveyed to a downstream finishing stage for further monomer stripping, drying and/or pelletizing to form a plasticized polymer product blend. Suitable plasticizers for in-line production and blending include polyalphaolefin oligomers, polybutenes, low glass transition temperature polymers and combinations thereof.