Date: 201906
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Time Pressure; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Normal take off from Runway XXR at ZZZ with First Officer flying. Fairly heavy B737NG at 174;000 lbs; 35;000 lbs of fuel bound for ZZZ1. At approximately 2;000 MSL I began to detect a burning or melting plastic odor; smelling identical to what a piece of Tupperware that had fallen onto the heating element in a dishwasher and melted would smell like. I asked the First Officer if he smelled it and he replied 'YES'. We were still sterile cockpit; but the flight attendants called from the back and were very concerned with the same odor that was 'filling the cabin.' I told them to search the cabin to see if the source of the odor could be isolated; lavatories; ovens; coffee maker etc.; and call back. We had leveled at 5;000 feet and when ZZZ Departure issued us another climb; I declined the climb and told ATC we had an issue and would get back with them. At this point the First Officer's audio control panel went haywire with extremely loud static being broadcast into his headset and speaker; regardless of what radio was selected and he was unable to transmit. The Purser called back and said that the source of the odor could not be determined but it was strong; and she now smelled what she described as 'burning paper.' This aircraft had just returned to service after a change of the #2 engine and this was the first flight after the change. The engine in terms of performance was operating normally but the possibility of it being involved in creating this situation did give me further concern. With the combination of the unidentifiable odor and failure of communication equipment; I made the decision to [advise ATC] and return back to ZZZ.Once [advised]; ATC was very efficient in getting us back around for a landing on one of the long western facing runways and things happened rapidly. We were landing overweight and I wanted a long runway for braking and winds which were quite gusty with a crosswind component. The First Officer remained as the Flying Pilot and executed a normal approach and landing to Runway XXR. Upon exiting the runway; we had fire and emergency personnel inspect the aircraft for a potential fire source. No abnormal heat source could be detected; and we returned to [the] gate without further incident.


Second narrative: I was the Pilot Flying taking off from Runway XXR out of ZZZ when passing thru 2;500 feet we started smelling a plastic like burning smell. The Captain and I were discussing the odor when the flight attendant called us and told of the same type of odor taking over the cabin. At this time; the Captain was talking to the Flight Attendant and I was flying and working the radios. At 5;000 feet we were given a climb and I told ATC to standby. Shortly after that; my radio comms 1 and 2 went completely to static. At that point; the Captain [advised ATC] and wanted vectors for an immediate return. We worked through the checklists and set up for an ILS to [Runway] XXR due to the strong crosswinds and overweight landing we wanted a long runway. ATC helped out with getting us back and notifying the emergency equipment to be ready when we touched down. On landing and rollout everything was normal. The emergency personnel inspected the outside of the aircraft for any signs of fire and smoke and gave us the OK to continue back to gate. On a side note; the airplane had come from the hangar and the number 2 engine had been replaced.


Synopsis: B737NG flight crew reported a return to departure airport due to smelling a 'burning plastic' odor throughout the aircraft during initial climb.