Equinor shut the plant on Norway's west coast in February to install a mercury removal unit. It was previously expected to restart by the end of April.
"We are planning for a safe (production) start-up... We expect this to take around four weeks from now," said a spokesperson.
Tjeldbergodden has an annual methanol production capacity of around 900,000 tonnes, accounting for about a quarter of Europe's total, according to Equinor.
Equinor has 82.01% stake in the plant and ConocoPhillips owns the remaining 17.99%.
(Reporting by Nerijus Adomaitis, editing by Gwladys Fouche)