General:
* Consolidate
* Integrate
* Combine
* Amalgamate
* Coalesce
* Harmonize
* Conjoin
* Unite
* Merge
More specific:
* Centralize (if referring to power or authority)
* Standardize (if referring to processes or systems)
* Reconcile (if referring to differences or opposing viewpoints)
The best synonym depends on the specific situation and the nuance you want to convey.