When the status shows "Required reviews completed," it means that the reviews have come in, but it might not mean that your paper is being evaluated by the editor. Generally, big journals such as Elsevier receive a large number of submissions and the editor has to handle multiple submissions. It is possible that your paper has not yet gone to the editor, since there are other papers that have come in before yours. Your paper is possibly still waiting to be checked by the editor. Once the editor starts checking it, the status generally shows "With editor" or "Evaluating recommendation."

While it is not uncommon for this stage to take more than a month, it would be a good idea to write to the editor as this sometimes helps quicken the process. You email can simply ask what the current status of the paper is and by when you can expect a decision.

We have a handbook on templates for communicating with the journal which includes a template for inquiring about the status of a paper. 

Related reading:

Tracking your manuscript status in journal submission systems