Systems and methods for running and cementing casing into wells drilled with dual-gradient mud systems include running casing through a subsea wellhead connected to a marine riser, the casing having an auto-fill float collar, and connecting a landing string to the last casing run. The landing string includes a surface-controlled valve (SCV) and a surface-controlled ported circulating sub (PCS). The SCV and PCS are manipulated as needed when running casing, washing it down while preventing u-tubing on connections and prior to cementing to displace mixed density mud from the landing string and replace it with heavy-density mud prior to circulating below the mudline thus maintaining the dual gradient effect.