A thermoplastic polymer composition comprises a polyethylene polymer composition and a salt of bicyclo[2.2.1]heptane-2,3-dicarboxylic acid. The polyethylene polymer composition can have a Melt Relaxation Product of 50,000 or less. A method for producing an injection molded article comprises the steps of (a) providing a thermoplastic polymer composition as described above, (b) melting the thermoplastic polymer composition, (c) injecting the molten thermoplastic polymer composition into a mold cavity, (d) cooling the molten thermoplastic polymer composition, and (e) ejecting the injection molded article from the mold cavity. A method for producing a film comprises the steps of (a) providing a thermoplastic polymer composition as described above, (b) melting the thermoplastic polymer composition, (c) extruding the molten thermoplastic polymer composition through a slot-shaped die orifice to form a film, (d) cooling the film, and (e) collecting the film.