General synonyms:
* Deception
* Concealment
* Disguise
* Masking
* Pretense
* Feigning
* Hypocrisy
* Duplicity
More specific synonyms:
* For hiding true feelings: Suppression, repression, inhibition
* For hiding one's true intentions: Deceit, fraud, trickery
* For deliberately misleading: Misrepresentation, misdirection, fabrication
The best synonym depends on the context in which you are using the word "dissimulation."