Cut kielbasa to bite-size pieces. Add to boiling water. Boil 8 to 10 minutes; poke with fork to release grease from meat. In slow cooker/Crock Pot, melt jelly on low heat. Add mustard. When kielbasa is done boiling, rinse, then add to slow cooker/Crock Pot mixture. Coat all of the meat. Simmer for 1 hour or more on low.