Nouns that capture the feeling:
* Self-satisfaction: This is a good general term for the feeling of smugness, especially when it comes from achieving something.
* Complacency: This describes a state of contentment with oneself, sometimes to the point of being unaware of potential problems.
* Arrogance: This emphasizes the overconfidence and sense of superiority associated with smugness.
* Conceit: This focuses on the inflated sense of self-importance that underlies smugness.
Nouns that capture the attitude:
* Smugness: This is the most direct noun equivalent of the adjective "smug."
* Patronizing: This describes a condescending attitude, which can be a way to express smugness.
* Superiority: This emphasizes the belief that one is better than others.
* Egotism: This refers to excessive self-importance and often accompanies smugness.
The best choice will depend on the specific situation and the nuance you want to convey.