An unexpected delay at Norway’s Nyhamna gas processing plant this month resulted in a dip in Norway’s gas production and higher European prices as traders adjusted to Shell plc’s postponed planned maintenance.

The plant had been shut down since May 19 and was originally slated to resume production on June 21, but maintenance work on the processing plant was halted after discovering gas in a cooling system. The plant now has a planned start-up date scheduled for July 15, Shell said last week.

European gas prices have jumped over 30% since the start of June, after falling more than 60% in the first five months of the year. Other planned outages in Norway for later in the summer and an unplanned outage at the Norne gas field scheduled to end Saturday have added to supply...