Word2vec tends to indicate similar words – but as you've probably seen, the kind of similarity it learns includes more than just pure synonyms.
For example, word2vec similarities include words that appear in similar contexts, such as alternatives including even opposites. (After all, 'hot' and 'cold' are very similar words in many ways – both adjectives, primarily relating to temperature, and also figuratively used in the same sorts of non-temperature contexts. Only in their contrast are they antonyms.)
You'll likely need to use other techniques (perhaps in concert with word2vec) for pure-synonym discovery.