Sometimes, when delicious food comes up in a conversation, or when recalling a delicious item of cuisine you've previously eaten, the common phrase "nom nom nom" doesn't fully express the extent to which you were pleased by the food. When this occurs, usage of the phrase "nom city" is appropriate, obviating the need to repeat "nom" excessively to get your point across.
Example 1: "Dude, I had the best chinese food last night. Nom city."
Example 2: "Have you tried the cheesesteaks at the pizza place down the street?" "Yeah man, nom city."