Antonyms:
* Avoidably
* Possibly
* Contignently
* Optionally
* Avoidably
* Uncertainly
* Unpredictably
* Doubtfully
* By chance
Synonyms:
* Certainly
* Surely
* Undeniably
* Inescapably
* Unquestionably
* Without fail
* Bound to
* Doomed to
* For sure
The best choice will depend on the specific context of your writing.