Make sure your refrigerator is unplugged before working on it. From the description of your problem it sounds like the refrigerator is not going through the automatic defrost cycle. There are three defrost components on your model, the defrost thermostat, control board, and defrost heater. Replacing the defrost thermostat and control board will take care of the problem 99% of the time on these models. The heater is not very common to fail on this model. You can test the heater by using an ohm meter to check it for continuity. The defrost thermostat is located behind the back panel in the freezer section, and it is clipped onto the evaporator coil on the top right side. You can test it by checking for continuity while the thermostat is cold (still covered in frost). You will need to remove everything from the freezer to access this. While the freezer is apart, make sure you fully defrost the evaporator coil. If both the defrost heater and defrost thermostat are good, then you have a bad control board. We recommend also replacing the defrost thermostat at the same time as the control board to ensure you don't have more problems down the road. Let me know what you find, and how the repair goes.
These are the parts for your refrigerator:
Control Board - PD00004512 (mfg# W10503278)
Defrost Thermostat - PD00003442 (mfg# W10165425)